It is strange that no matter how I change the size_limit in machine #1, the uebimiau webmail always shows the the maximum mailbox size in 5120Kb (5MB?),while in machine #2,uebimiau webmail always gives the right quota limit of ISPConfig user's mailspace setting.

If i remember correctly, the mailbox size that UebiMiau uses is either defined in the uebiMiau config file or read from the ISPConfig database. Maybe you did not set a mailbox size limit on the second server for the mail user?

It is strange that no matter how I change the size_limit in machine #1, the uebimiau webmail always shows the the maximum mailbox size in 5120Kb (5MB?),while in machine #2,uebimiau webmail always gives the right quota limit of ISPConfig user's mailspace setting.